This invention relates to printing presses, and, more particularly, a method and apparatus for exchanging the rolls of a printing press.
Printing presses such as flexographic presses include one or more decks for supporting rolls adjacent a central impression (CI) drum or cylinder. For example, a flexographic press typically includes multiple color decks, and each color deck includes a plate roll and an anilox roll. The anilox roll transfers ink from an ink fountain or doctor chamber to the plate roll. The plate roll carries the print image and imprints the image onto a web which is supported by the central impression (CI) drum.
The plate roll and anilox roll of certain designs of flexographic presses comprise a mandrel and a sleeve which is removably mounted on the mandrel. The outer surface of the sleeve of the plate roll carries the print image. The outer surface of the sleeve of the anilox roll is coated with ceramic and is engraved for controlled pick up of ink and transfer of ink to the plate roll.
In some flexographic presses, both ends of the plate and anilox rolls are supported in bearings on the front and back frames of the press. In order to exchange a roll or the sleeve of a roll, the front bearing or bearing housing is removed from the front end of the roll, and the back end of the roll is supported as a cantilever in the back bearing. However, the front end of the roll is inside of the front frame, and the operator must reach into the frame to remove the roll or to remove the sleeve of a roll. Two problems are thereby presented--potential operator injury and potential damage to the outer surface of the roll.
The space between the OD (outside diameter) of the roll (plate or anilox) and the machine framework is generally close. Typical clearances for maximum roll diameters are less than 1/4 inch. The operator's hands must reach for the roll and pull outward. This can cause his hands to bump the machine framework or pinch between the roll and the machine framework.
Roll damage occurs most often when inserting a sleeve onto an empty mandrel. The surface of the sleeve bumps and/or rubs on the machine before it is placed on and then guided by the mandrel. The surface of the plate roll sleeve carries the print image and is therefore susceptible to damage. This leads to expensive machine downtime. The surface of the anilox sleeve is coated with ceramic and engraved. If the ceramic surface gets even minor chips, the roll must be coated and engraved again.
Another problem with the present art is the bearing cap. The mandrel (or roll journal) must be supported in a bearing cap (or housing) on both ends. The present art has these caps automated to open or release the mandrel. This automation adds cost to the machine. Automation also requires compliance, which reduces stiffness. The reduced stiffness leads to print bounce problems.
In other designs of flexographic presses, only one end of the plate roll and the anilox roll is supported by a bearing during printing. The rolls are therefore cantilevered both while the press is running and while exchanging rolls or sleeves. The problem with machines in which the rolls are cantilevered during running is poor stiffness. The intermittent loading from the printing nip cannot be handled adequately by a mandrel supported only on one end. On a typical flexographic printing press having a width from 29 inches to 65 inches, the print speed would be greatly reduced to avoid print skipping problems.